Nickel powder and conductive paste

ABSTRACT

The present invention relates to nickel powder characterized in that the rate of the nickel particles whose particle size is not less than 1.2 time the average particle size as determined by the observation with an SEM is not more than 5% of the total number of nickel particles, that the rate of nickel particles whose particle size is not more than 0.8 time the average particle size is not more than 5% of the total number of nickel particles and that the tap density of the nickel powder is not less than 2.5 g/cm 3 , as well as a conductive paste for use in making a multilayer ceramic capacitor. The nickel powder of the present invention has a narrow particle size distribution and an excellent packing ability in a conductive paste and is particularly suitably used for forming a thin and uniform inner electrode for a multilayer ceramic capacitor, without being accompanied by the formation of any crack and/or the generation of any delamination.

TECHNICAL FIELD

The present invention relates to nickel powder and a conductive pastefor use in making a multilayer ceramic capacitor and more particularlyto nickel powder, which has a narrow particle size distribution and anexcellent packing ability in a conductive paste and which is, inparticular, suitably used for forming a thin and uniform inner electrodefor a multilayer ceramic capacitor without being accompanied by theformation of any crack and the generation of any delamination as well asa conductive paste for use in making a multilayer ceramic capacitor,which comprises the foregoing nickel powder.

BACKGROUND ART

A multilayer ceramic capacitor comprises a plurality of layers of aceramic dielectric substance and a plurality of inner electrode layers,which are alternately multilayer and united and such a multilayerceramic capacitor is in general produced by preparing a conductive pasteby converting metal fine powder as an inner electrode material into apaste, printing a green sheet of a ceramic dielectric substance with theresulting conductive paste, laminating a plurality of the printed greensheets in such a manner that the ceramic dielectric green sheet and theconductive paste are arranged alternately, pressing the laminatedprinted green sheet with heating to thus unite them, and then firing theresulting assembly at a high temperature in a reducing atmosphere tothus unify the ceramic dielectric layers and the inner electrode layers.

As the inner electrode material, there has conventionally been used, forinstance, a precious metal such as platinum, palladium orsilver-palladium, but there have recently been developed techniques,which make use of base metals such as nickel in place of precious metalssuch as platinum, palladium and silver-palladium in order to save theproduction cost and these techniques have been advanced. However, if anickel powder-containing paste is used for forming an inner electrode,problems such as the formation of cracks and/or the generation ofdelamination arise.

Moreover, electronic parts produced by using conductive pastes such asmultilayer ceramic capacitors have recently been more and moreminiaturized and the ceramic dielectric layer and the inner electrodelayer have correspondingly been more and more thinner and the number ofthese layers multilayer has been increased. Accordingly, there havepresently been produced a multilayer part such as a multilayer ceramiccapacitor in which the thickness of the dielectric layer is not morethan 2 μm, the thickness of the inner electrode layer is not more than1.5 μm and the number of layers multilayer is not less than 100. Toproduce such a part, however, it is necessary to form a thin and uniforminner electrode without being accompanied by the formation of any crackand/or the generation of any delamination.

To obtain a thinner inner electrode layer, it may be satisfactory to usemetal fine powder having a small average particle size balanced with thethickness of the inner electrode layer. However, coarse particles may bepresent in such metal fine powder even if the average particle size ofthe powder falls within the desired range. Accordingly, if an innerelectrode layer is formed by using a conductive paste containing suchmetal fine powder, the coarse particles present therein may formprojections on the resulting inner electrode layer, the projections mayin turn break through the thin ceramic dielectric layer to thus form ashort-circuit between the neighboring inner electrode layers.Accordingly, to prevent the formation of any short-circuit between innerelectrode layers, it is necessary to use metal fine powder having anaverage particle size substantially smaller than that balanced with thethickness of such a thin inner electrode layer.

For instance, Japanese Un-Examined Patent Publication No. Hei 11-189801discloses nickel ultra-fine powder whose average particle size rangesfrom 0.2 to 0.6 μm and in which the rate of coarse particles having aparticle size of not less than 2.5 times the average particle size isnot more than 0.1% based on the number of particles and furtherdiscloses in the fourth column, lines 21 to 24 that “If the particlesize of coarse particles is limited to, for instance, about 1.5 μm, theaverage particle size of the ultra-fine nickel powder according to thepresent invention should accordingly be limited to 0.6 μm”. Thus, it isnecessary to use metal fine powder having a considerably small averageparticle size in order to produce a thin inner electrode layer.

In addition, to ensure stable conductivity of the resulting electrodes,such nickel fine powder should not only be finer, but also should have ahigh packing ability in a vehicle used in the production of a conductivepaste. However, it may be more and more difficult to improve the packingability of such fine particle-containing nickel powder in the conductivepaste and the viscosity of the resulting conductive paste increases, asthe particle size of the fine particles is reduced. In addition, aproblem arises such that the heat shrinkage of the conductive pastelayer and the oxidation of the nickel powder included therein areaccelerated upon firing the printed green sheet.

It is thus an object of the present invention to provide nickel powder,which has a narrow particle size distribution, which has an excellentpacking ability in a conductive paste and which can particularlysuitably be used for forming a thin and uniform inner electrode for usein making a multilayer ceramic capacitor without being accompanied bythe formation of any crack and/or the generation of any delamination, aswell as a conductive paste, which is used for the manufacture of amultilayer ceramic capacitor.

DISCLOSURE OF THE INVENTION

The inventors of this invention have conducted various studies toaccomplish the foregoing objects, have found that if the rates of coarseparticles and fine particles present in nickel powder are reduced torelatively low levels and the tap density of the nickel powder is higherthan a predetermined value, an inner electrode layer free of anyprojection can be formed without unnecessarily reducing the particlesize of the nickel powder and the formation of any short circuit betweenthe neighboring inner electrodes of the resulting ceramic capacitor cancertainly be inhibited, that the oxidation of nickel particles and theheat shrinkage of the electrode layer can be inhibited because of thelow content of fine particles and that the nickel powder has a highpacking ability in a conductive paste due to a high tap density of thepowder and also permits the formation of a beautiful sintered film andthus have completed the present invention.

Moreover, the inventors have also found that if the particle size ofcrystallites present in each nickel particle is small in addition to theforegoing characteristics, the sintering of nickel particles gentlyproceeds, the sintering speed is uniform and as a result, a thin anduniform inner electrode can be formed without being accompanied by theformation of any crack and/or the generation of any delamination.

Accordingly, the nickel powder of the present invention is characterizedin that the rate of the nickel particles whose particle size is not lessthan 1.2 time the average particle size as determined by the observationwith an SEM is not more than 5% of the total number of nickel particles,that the rate of nickel particles whose particle size is not more than0.8 time the average particle size is not more than 5% of the totalnumber of nickel particles and that the tap density of the nickel powderis not less than 2.5 g/cm³.

The present invention also relates to a conductive paste for use inmaking a multilayer ceramic capacitor, which is characterized bycomprising nickel powder having the foregoing characteristic properties.

BEST MODE FOR CARRYING OUT THE INVENTION

It is important in the nickel powder of the present invention that has arate of the nickel particles whose particle size is not less than 1.2time the average particle size as conveniently determined by theobservation by an SEM with a magnification of ×10000 is not more than5%, preferably not more than 4% and most preferably not more than 3% ofthe total number of nickel particles and that a rate of nickel particleswhose particle size is not more than 0.8 time the average particle sizeis not more than 5%, preferably not more than 4% and most preferably notmore than 3% of the total number of nickel particles. In other words,the rate of coarse particles present in the nickel powder is relativelylow and thus the particle sizes of individual nickel particles presentin the nickel powder are considerably uniform. Therefore, each innerelectrode layer used in, for instance, a multilayer ceramic capacitorproduced by using a conductive paste, which contains such nickel powder,may have a considerably thin thickness as compared with the averageparticle size of the nickel powder and may have a surface almost free ofany projection. Accordingly, the resulting multilayer ceramic capacitornever causes any short circuit between the neighboring inner electrodes.In addition, the nickel powder of the present invention has a relativelylow rate of fine particles and therefore, the paste containing suchnickel powder never suffers from such problems that the paste has a highviscosity and that the heat shrinkage of the paste layer and theoxidation of the nickel particles included therein are acceleratedduring firing the printed green sheet.

It is also important in the nickel powder of the present invention thatit has a tap density of not less than 2.5 g/cm³, preferably not lessthan 2.7 g/cm³ and more preferably not less than 3.0 g/cm³. In otherwords, the nickel powder has a high packing ability in a conductivepaste prepared by using the same because of its high tap density andbeautiful sintered films can be obtained in, for instance, a multilayerceramic capacitor prepared by using such a nickel powder-containingconductive paste.

Moreover, in the nickel powder according to the present invention, theaverage particle size of crystallites present in each nickel particle ispreferably less than 400 Å and more preferably not more than 300 Å.Beautiful sintered films can be obtained in, for instance, a multilayerceramic capacitor prepared by using a conductive paste containing suchnickel powder. In addition, the particle size of crystallites present ineach nickel particle is small and therefore, the sintering of nickelparticles gently proceeds. More specifically, the sintering may proceedat a breath if the particle size of crystallites present in each nickelparticle is large, while if the particle size of crystallites present ineach nickel particle is small, the sintering is first taken placebetween crystallites within every particles, then particles graduallyundergo sintering and therefore, the sintering speed is uniform. Thisaccordingly results in the formation of beautiful films and theresulting product hardly undergoes any crack-formation and/or anydelamination.

As has been discussed above, if the nickel powder according to thepresent invention is one in which the rate of the nickel particles whoseparticle size is not less than 1.2 time the average particle size asdetermined by the observation with an SEM is not more than 5% of thetotal number of nickel particles, the rate of nickel particles whoseparticle size is not more than 0.8 time the average particle size is notmore than 5% of the total number of nickel particles, the tap density ofthe nickel powder is not less than 2.5 g/cm³ and the average particlesize of crystallites present in each nickel particle is less than 400 Å,a quite uniform, compact and thin inner electrode film can be formedwithout forming any crack and/or generating any delamination, while theformation of projections on the surface of the inner electrode layer ismore completely inhibited, in a multilayer ceramic capacitor or the likeprepared by using a conductive paste containing such nickel powder.

Regarding the nickel powder of the present invention, a multilayerceramic capacitor is produced by using a conductive paste containing theforegoing nickel powder, the average particle size of the powder asdetermined by the observation by an SEM preferably ranges from 0.1 to 1μm and more preferably 0.2 to 0.6 μm.

As has been described above, the nickel powder according to the presentinvention has a narrow particle size distribution and an excellentpacking ability in a conductive paste and therefore, the conductivepaste containing the nickel powder can be used in a variety ofapplications. In particular, when the nickel powder is used for formingthe inner electrode of a multilayer ceramic capacitor, a quite uniform,compact and thin inner electrode film can be produced by using thenickel fine powder without unnecessarily reducing the particle sizethereof, without causing any crack formation and/or generating anydelamination, while the formation of projections on the surface of theinner electrode layer is more completely inhibited.

In addition, the conductive paste for use in making a multilayer ceramiccapacitor according to the present invention is characterized bycomprising the foregoing nickel powder according to the presentinvention and the conductive paste for use in making a multilayerceramic capacitor of the present invention is particularly suitably usedfor forming a thin, uniform inner electrode, since the paste comprisesthe nickel powder having the foregoing excellent characteristicproperties.

Then a preferred method for the production of the nickel powder of thepresent invention will be described below.

The nickel powder of the present invention can be produced by adding anaqueous nickel salt solution containing a nickel complex to an aqueousalkali metal hydroxide solution to thus prepare a nickelhydroxide-containing slurry, bringing the resulting slurry into contactwith a hydrazine reducing agent under the temperature condition of notless than 55° C. to reduce the nickel hydroxide into nickel, subjectingthe resulting nickel powder to a disaggregation treatment and thensubjected to pneumatic classification to remove coarse particles andfine particles.

Such a nickel complex-containing aqueous nickel salt solution can beprepared by dissolving a nickel salt and a nickel complex-formingcompound preferably a water-soluble compound carrying a carboxyl groupand/or an amino group in water in any order; dissolving a water-solublecompound carrying a carboxyl group and/or an amino group in an aqueousnickel salt solution; or dissolving a nickel salt in an aqueous solutionof a water-soluble compound carrying a carboxyl group and/or an aminogroup. In short, it is necessary in the present invention to prepare anaqueous solution containing a nickel salt and a water-soluble compoundcarrying a carboxyl group and/or an amino group. In this respect,however, it is preferred to prepare the nickel complex-containingaqueous nickel salt solution by dissolving a water-soluble compoundcarrying a carboxyl group and/or an amino group in an aqueous nickelsalt solution. This is because, this method permits the easy formationof the desired nickel complex. Moreover, if using a nickelcomplex-containing aqueous nickel salt solution thus prepared, theresulting nickel particles have a uniform particle size and an improvedpacking ability in a conductive paste.

Specific examples of the foregoing water-soluble compound carrying acarboxyl group and/or an amino group include ethylenediaminetetraaceticacid, acetic acid, oxalic acid, malonic acid, salicylic acid,thioglycolic acid, glycine, ethylenediamine, alanine, citric acid,glutamic acid, lactic acid, malic acid, tartaric acid andtriethanolamine.

When the nickel complex is formed in the aqueous nickel salt solution,the amount of the water-soluble compound carrying a carboxyl groupand/or an amino group to be added to the aqueous solution ranges from0.005 to 0.5 and more preferably 0.01 to 0.1 as expressed in terms ofthe molar ratio with respect to the nickel salt present in the aqueousnickel salt solution. The use of the water-soluble compound in such anamount permits the production of nickel powder having a narrowerparticle size distribution and simultaneously permits the achievement ofwell-balanced cost performance.

As nickel salts usable in the foregoing preferred production method,there can be listed, for instance, nickel sulfate, nickel nitrate andnickel halide such as nickel chloride; examples of alkali metalhydroxides include sodium hydroxide and potassium hydroxide; andexamples of hydrazine reducing agents usable herein are hydrazine,hydrazine hydrate, hydrazine sulfate, hydrazine carbonate and hydrazinehydrochloride.

The concentration of the aqueous nickel salt solution used in thepreparation of the foregoing nickel hydroxide-containing slurrypreferably ranges from 10 to 150 g/L and more preferably 50 to 150 g/Las expressed in terms of the nickel ion concentration. The use of anaqueous nickel salt solution having such a concentration permits theachievement of such a narrow particle size distribution peculiar to thenickel powder of the present invention and simultaneously leads to theachievement of preferred results such as good production efficiency.

The concentration of the aqueous alkali metal hydroxide solution used inthe preparation of the foregoing nickel hydroxide-containing slurrypreferably ranges from 20 to 300 g/L and more preferably 60 to 250 g/L.In addition, the amount of the aqueous nickel salt solution relative tothat of the aqueous alkali metal hydroxide solution is such that theamount of the alkali metal hydroxide present in the aqueous alkali metalhydroxide solution is preferably 1.1 to 2 eq. and more preferably 1.3 to1.8 eq. per one eq. of the nickel salt present in the aqueous nickelsalt solution. It is thus preferred to use these ingredients in suchrelative amounts to ensure the stable formation of nickel hydroxide andto simultaneously achieve well-balanced cost performance.

In the foregoing preferred production method, it is very important thatthe nickel hydroxide-containing slurry is brought into contact with ahydrazine reducing agent at a temperature of not less than 55° C. tothus reduce the nickel hydroxide into elemental nickel. If the reductionis carried out at a temperature of less than 55° C., it is difficult toobtain nickel powder having a uniform particle size and the resultingpowder comprises a large amount of coarse nickel particles. Moreover,alkali metals as impurities are mixed in the resulting nickel powder ina high rate. Therefore, the reaction temperature during the reduction ofthe nickel hydroxide is set at a level of not less than 55° C. andpreferably not less than 60° C. in the foregoing preferred productionmethod.

Moreover, the nickel powder produced by bringing the foregoing nickelhydroxide into contact with an aqueous hydrazine solution maintained ata temperature of not less than 55° C., preferably not less than 60° C.to thus reduce the nickel hydroxide has a uniform particle size asdetermined by the observation with an SEM and the total content ofimpurities derived from the raw materials for the reaction in theresulting nickel powder is reduced to an extremely low level.

When preparing the nickel powder according to the present invention, itis important to subject the nickel powder obtained after the reducingreaction to a disaggregation or disintegration treatment. This isbecause the nickel powder per se obtained through a wet reaction isstrongly agglomerate and therefore, the nickel powder suffers from thesame disadvantages as those described in the foregoing section entitled“Background Art” observed when the powder contains coarse particles. Inaddition, the nickel powder also has a low tap density and therefore,the powder has an insufficient packing ability in a conductive paste.For this reason, the disadvantages such as those observed when suchnickel powder is used in, for instance, a conductive paste cannoteffectively be eliminated.

This disaggregation treatment is not restricted to any particular one,but may, for instance, be a high speed rotary impaction-pulverizationtreatment wherein nickel powder is pulverized by leading the powder tocollide with a rotatable part rotating at a high speed; a media-stirringpulverization treatment in which nickel powder is stirred together with,for instance, beads to thus pulverize the same; and a high hydraulicdisaggregation treatment and a jet impaction treatment in which two jetsof nickel particle-containing slurry are led to collide with each otherfrom different two directions at a high hydraulic pressure to thuspulverize the same.

Examples of devices for carrying out such disaggregation treatments area high speed moving body-impaction type gas flow pulverizer, animpaction type pulverizer, a cage mill, a medium-stirring type mill, anaxial flow mill, and a jet-impaction device. More specifically, theremay be listed, for instance, Super Hybrid Mill (available fromIshikawajima-Harima Heavy Industries Co., Ltd.), Jet Mill (availablefrom Ebara Corporation), Super Mass Colloider (available from MasukoSangyo Co., Ltd.), Beads Mill (available from Irie Shokai Co., Ltd.),Altimizer (available from Sugino Machine Mfg. Co., Ltd.), NC Mill(available from Ishii Pulverizer Mfg. Co., Ltd.), Disintegrator(available from Otsuka Tekko Co., Ltd.), ACM Pulverizer (available fromHosokawa Micron Co., Ltd.), Turbo Mill (available from Matsubo Co.,Ltd.), Super Micron (available from Hosokawa Micron Co., Ltd.), Micross(available from Nara Machine Mfg. Co., Ltd.), New Cosmomyzer (availablefrom Nara Machine Mfg. Co., Ltd.), Fine Victor Mill (available fromHosokawa Micron Co., Ltd.), Ecoprex (available from Hosokawa Micron Co.,Ltd.), CF Mill (available from Ube Industries, Ltd.), Hybridizer(available from Nara Machine Mfg. Co., Ltd.), Pin Mill (available fromAlpinae Co., Ltd.), Pressure Homogenizer (available from NipponPrecision Machine Mfg. Co., Ltd.), Harrel Homogenizer (available fromKokusan Seiko Co., Ltd.), Mechano Fusion System (available from HosokawaMicron Co., Ltd.) and Sand Mill (available from Yodo Casting Co., Ltd.).

Moreover, it is also important to subject the nickel powder to apneumatic classification before and/or the disaggregation treatment tothus remove coarse particles and fine particles when preparing thenickel powder according to the present invention. This is because if thenickel powder is subjected to a pneumatic classification prior to thedisaggregation treatment, highly agglomerate particles and hugeparticles can be removed in advance, while if the nickel powder issubjected to a pneumatic classification after the disaggregationtreatment, fine particles may likewise be removed in addition to highlyagglomerate particles and huge coarse particles, which have stillremained in the nickel powder even after the disaggregation treatment.This treatment for removing the foregoing particles may be carried outat any appropriate stage depending on the quality of the nickel powderto be prepared and it is a matter of course that this classificationtreatment may be carried out both before and after the disaggregationtreatment.

Regarding this treatment for removal, the use of, for instance, a sieveclassifier is not suitable, while taking into consideration theprevention of any oxidation of the nickel powder surface andworkability. For this reason, most preferred means is a pneumaticclassifier and more specifically, preferably used are, for instance, anair separator as a centrifugal classifier, Spedick Classifier, Aquecut,Turbo Classifier.

Then a preferred method for the preparation of the conductive paste foruse in making a multilayer ceramic capacitor will be described below.

The conductive paste of the present invention for use in making amultilayer ceramic capacitor is composed of, for instance, theaforementioned nickel powder of the present invention, a resin and asolvent. More specifically, resins usable herein are, for instance,cellulose derivatives such as ethyl cellulose, vinyl non-curing resinssuch as acrylic resins, polyvinyl butyral resins and polyvinyl alcohols,and thermosetting resins preferably used in combination with peroxidessuch as epoxy and acryl. In addition, solvents usable herein are, forinstance, terpineol, tetralin, butyl carbitol and carbitol acetate,which may be used alone or in any combination. The paste may, ifnecessary, comprise glass frits. The conductive paste of the presentinvention used in the production of a multilayer ceramic capacitor maybe prepared by mixing and stirring the foregoing ingredients by using amixing device such as a ball mill or a three-roll mill.

The present invention will more specifically be described in more detailbelow with reference to the following Examples and Comparative Examples.

EXAMPLES 1

An aqueous solution prepared by dissolving 44.8 kg of nickel sulfatehexahydrate (grade: 22.2% by mass) and 1.8 kg of citric acid monohydratein 80 L of pure water was slowly dropwise added to 100 L of a 200 g/Laqueous sodium hydroxide solution while maintaining the temperature ofthe solutions at 60° C. to thus separate out nickel hydroxide. Then tothe resulting suspension, there was gradually added 30 kg of hydrazinemonohydrate over 30 minutes, while maintaining the temperature of thesolutions at 60° C., to thus reduce the nickel hydroxide into nickel,followed by washing the resulting nickel powder with pure water till thepH value of the wash liquid was reduced to a level of not more than 9,filtration, drying and then subjecting the resulting powder to adisaggregation treatment in Pulverizer AP-1SH Model (available fromHosokawa Micron Co., Ltd.) at a rotational speed of 2500 rpm to thusgive nickel powder. Then coarse particles were removed from thisdisintegrated nickel powder by treating the nickel powder in an airseparator: SF Sharp Cut Separator KSC-02 Model (available from Kurimoto,Ltd.) at a rotor rotational number of 6000 rpm and an air flow rate of7.2 m³/min to thus give desired nickel powder.

The resulting nickel powder was observed by an SEM with a magnificationof ×10000 to determine the particle sizes of 1500 particles, in all,present in 5 visual fields randomly selected. As a result, the averageparticle size of these particles was found to be 0.51 μm, the number ofparticles whose particle size was greater than 0.61 μm (0.51×1.2=0.612)was found to be 30 (corresponding to 2.0% of the total number ofparticles examined) and the number of particles whose particle size wasless than 0.41 μm (0.51×0.8=0.408) was found to be 40 (corresponding to2.7% of the total number of particles examined). In addition, the tapdensity of the nickel powder was found to be 3.23 g/cm³ and the averageparticle size of the crystallites present in each particle was found tobe 195 Å.

Moreover, to 100 parts by mass of the foregoing nickel powder, there wasadded a binder, which comprised 8 parts by mass of ethyl cellulose, 100parts by mass of terpineol and 12 parts by mass of butyl carbitol,followed by mixing these ingredients and kneading them in a roll mill togive a conductive paste. The conductive paste thus prepared was printedon a polyimide (PI) film (UPILEX, available from Ube Industries, Ltd.)having a thickness of 125 μm through a Tetoron screen mask of 380-meshsize (printed pattern: 4 cm×4 cm). The printed PI film was subjected toa leveling treatment for 15 minutes at room temperature and thenpre-drying in a thermostatic dryer with internal hot air circulation setat 60° C. for 30 minutes. Moreover, the printed PI film was transferredto a thermostatic dryer with internal hot air circulation set at 120° C.to thus subject the film to complete curing for 60 minutes. The PI filmthus treated was removed from the dryer, followed by allowing to cooldown to room temperature, and then determination of film density atrandomly selected 20 points. As a result, the average film density wasfound to be 4.71 g/cm³ and the standard deviation thereof was found tobe 0.08 g/cm³.

An attached body was produced by using the foregoing conductive paste insuch a manner that the thickness of a dielectric layer was 2 μm, that ofan inner electrode layer was 1.5 μm and the number of multilayer layerswas 350. Pieces each having a size of 2.0 mm×1.25 mm were cut from theresulting attached body or the multilayer body, followed by drying andsubjecting the pieces to a treatment for the removal of the binder.Thereafter, the pieces were fired at 1200° C. in a hydrogen-nitrogenmixed gas to give multilayer ceramic capacitors having a size of 2.0mm×1.25 mm×1.25 mm. Then 200 samples were randomly taken from theresulting multilayer ceramic capacitors and were inspected for theformation of any crack and generation of any delamination. As a result,the number of rejects was found to be 2 and therefore, the percentage ofrejects was found to be 1%.

The foregoing results of the various determinations, calculated valuesand the results of the observations are summarized in the followingTable 1. In this connection, “Rate of ≧1.2 d” appearing in Table 1 meansthe rate of the particles whose particle size is not less than 1.2 timethe average particle size with respect to the total number of particlesand “Rate of ≦0.8 d” appearing in Table 1 means the rate of theparticles whose particle size is not more than 0.8 time the averageparticle size with respect to the total number of particles.

EXAMPLE 2

An aqueous solution prepared by dissolving 44.8 kg of nickel sulfatehexahydrate (grade: 22.2% by mass) and 0.65 kg of glycine in 80 L ofpure water was slowly dropwise added to 100 L of a 200 g/L aqueoussodium hydroxide solution while maintaining the temperature of thesolutions at 60° C. to thus separate out nickel hydroxide. Then to theresulting suspension, there was gradually added 42 kg of hydrazinemonohydrate over 20 minutes, while maintaining the temperature of thesuspension at 60° C., to thus reduce the nickel hydroxide into nickel.The resulting nickel powder was washed with pure water till the pH valueof the wash liquid was reduced to a level of not more than 9, followedby filtration and then drying. Thereafter, the resulting nickel powderwas subjected to a disaggregation treatment in Hybridizer NHS-3 Model(available from Nara Machine Mfg. Co., Ltd.) at a rotational speed of4000 rpm for 5 minutes. Then coarse particles were removed from thisdisintegrated nickel powder by treating the nickel powder in an airseparator: SF Sharp Cut Separator KSC-02 Model (available from Kurimoto,Ltd.) at a rotor rotational number of 6000 rpm and an air flow rate of7.2 m³/min to thus give desired nickel powder.

The resulting nickel powder was inspected for various characteristicproperties according to the same methods used in Example 1. A conductivepaste was likewise prepared by the same method used in Example 1 and thefilm density as an inner electrode was also determined by the samemethod used in Example 1. Furthermore, a multilayer ceramic capacitorproduced by the same method used in Example 1 was inspected for theformation of any crack and the generation of any delamination by thesame method used in Example 1. The foregoing results of the variousdeterminations, calculated values and the results of the observationsare summarized in the following Table 1.

EXAMPLE 3

An aqueous solution prepared by dissolving 44.8 kg of nickel sulfatehexahydrate (grade: 22.2% by mass) and 1.8 kg of citric acid in 80 L ofpure water was slowly dropwise added to 100 L of a 200 g/L aqueoussodium hydroxide solution while maintaining the temperature of thesolutions at 70° C. to thus separate out nickel hydroxide. Then to theresulting suspension, there was gradually added 42 kg of hydrazinemonohydrate over 30 minutes, while maintaining the temperature of thesuspension at 70° C., to thus reduce the nickel hydroxide into nickel.The resulting nickel powder was washed with pure water till the pH valueof the wash liquid was reduced to a level of not more than 9, followedby filtration and then drying. Then coarse particles were removed fromthis nickel powder by treating the nickel powder in a jet mill: EbaraTriade Jet PM100 Model (available from Ebara Corporation) at a rotorrotational number of 6000 rpm and an air flow rate of 7.2 m³/min to thusgive desired nickel powder.

The resulting nickel powder was inspected for various characteristicproperties according to the same methods used in Example 1. A conductivepaste was likewise prepared by the same method used in Example 1 and thefilm density as an inner electrode was also determined by the samemethod used in Example 1. Furthermore, a multilayer ceramic capacitorproduced by the same method used in Example 1 was inspected for theformation of any crack and the generation of any delamination by thesame method used in Example 1. The foregoing results of the variousdeterminations, calculated values and the results of the observationsare summarized in the following Table 1.

Comparative Example 1

An aqueous solution prepared by dissolving 44.8 kg of nickel sulfatehexahydrate (grade: 22.2% by mass) in 80 L of pure water was slowlydropwise added to 100 L of a 200 g/L aqueous sodium hydroxide solutionwhile maintaining the temperature of the solutions at 50° C. to thusseparate out nickel hydroxide. Then to the resulting suspension, therewas gradually added 42 kg of hydrazine monohydrate over 20 minutes,while maintaining the temperature of the suspension at 50° C., to thusreduce the nickel hydroxide into nickel. The resulting nickel powder waswashed with pure water till the pH value of the wash liquid was reducedto a level of not more than 9, followed by filtration and then drying.The resulting powder was then subjected to a disaggregation treatment inPulverizer AP-1SH Model equipped with a knife-type hammer (availablefrom Hosokawa Micron Co., Ltd.) at a rotational speed of 2500 rpm tothus give desired nickel powder.

The resulting nickel powder was inspected for various characteristicproperties according to the same methods used in Example 1. A conductivepaste was likewise prepared by the same method used in Example 1 and thefilm density as an inner electrode was also determined by the samemethod used in Example 1. Furthermore, a multilayer ceramic capacitorproduced by the same method used in Example 1 was inspected for theformation of any crack and the generation of any delamination by thesame method used in Example 1. The foregoing results of the variousdeterminations, calculated values and the results of the observationsare summarized in the following Table 1.

Comparative Example 2

An aqueous solution prepared by dissolving 44.8 kg of nickel sulfatehexahydrate (grade: 22.2% by mass) and 1.8 kg of citric acid monohydratein 80 L of pure water was slowly dropwise added to 100 L of a 200 g/Laqueous sodium hydroxide solution while maintaining the temperature ofthe solutions at 60° C. to thus separate out nickel hydroxide. Then tothe resulting suspension, there was gradually added 30 kg of hydrazinemonohydrate over 30 minutes, while maintaining the temperature of thesuspension at 60° C., to thus reduce the nickel hydroxide into nickel.The resulting nickel powder was washed with pure water till the pH valueof the wash liquid was reduced to a level of not more than 9, followedby filtration and then drying. The resulting powder was then subjectedto a disaggregation treatment in Pulverizer AP-1SH Model equipped with aknife-type hammer (available from Hosokawa Micron Co., Ltd.) at arotational speed of 2500 rpm to thus give desired nickel powder.

The resulting nickel powder was inspected for various characteristicproperties according to the same methods used in Example 1. A conductivepaste was likewise prepared by the same method used in Example 1 and thefilm density as an inner electrode was also determined by the samemethod used in Example 1. Furthermore, a multilayer ceramic capacitorproduced by the same method used in Example 1 was inspected for theformation of any crack and the generation of any delamination by thesame method used in Example 1. The foregoing results of the variousdeterminations, calculated values and the results of the observationsare summarized in the following Table 1.

Comparative Example 3

Sufficiently dried anhydrous nickel chloride (22.0 kg) having a sulfurcontent of 500 ppm was allowed to stand in a quartz container and thenevaporated with heating in a gas stream or argon gas as a carrier in aflow rate of 10 L/min while controlling the internal temperature of thecontainer to 900° C. Then hydrogen gas as a reducing gas was passedthrough the vaporized nickel chloride gas in a flow rate of 3.5 L/minwhile controlling the reducing temperature to 1000° C. to thus obtainnickel powder. The resulting nickel powder was washed with pure watertill the pH value of the wash liquid was reduced to a level of not morethan 9, followed by filtration and then drying. The resulting powder wasthen subjected to a disaggregation treatment in Pulverizer AP-1SH Modelequipped with a knife-type hammer (available from Hosokawa Micron Co.,Ltd.) at a rotational speed of 2500 rpm. Then coarse particles wereremoved from this disintegrated nickel powder by treating the nickelpowder in an air separator: SF Sharp Cut Separator KSC-02 Model(available from Kurimoto, Ltd.) at a rotor rotational number of 6000 rpmand an air flow rate of 7.2 m³/min to thus give desired nickel powder.

The resulting nickel powder was inspected for various characteristicproperties according to the same methods used in Example 1. In thisComparative Example 3, the nickel powder was prepared by a dry methodand therefore, it was found that the average particle size of thecrystallites present in each nickel particle was found to be large. Aconductive paste was likewise prepared by the same method used inExample 1 and the film density as an inner electrode was also determinedby the same method used in Example 1. Furthermore, a multilayer ceramiccapacitor produced by the same method used in Example 1 was inspectedfor the formation of any crack and the generation of any delamination bythe same method used in Example 1. The foregoing results of the variousdeterminations, calculated values and the results of the observationsare summarized in the following Table 1.

TABLE 1 Average Standard Average Rate Rate Particle Average DeviationNumber Particle of of Tap Size of Film of Film of % of Size (d) ≧1.2 d≦0.8 d Density Crystallite Density Density Rejects Rejects Example 10.51 μm 2.0% 2.7% 3.23 g/cm³ 195 Å 4.71 g/cm³ 0.08 g/cm³ 2 1.0% 2 0.44μm 2.6% 2.2% 2.85 g/cm³ 183 Å 4.73 g/cm³ 0.10 g/cm³ 3 1.5% 3 0.47 μm1.8% 2.6% 3.45 g/cm³ 238 Å 4.68 g/cm³ 0.11 g/cm³ 2 1.0% Comparative 10.59 μm 6.6% 5.2% 2.45 g/cm³ 173 Å 4.36 g/cm³ 0.30 g/cm³ 8 4.0% Example2 0.58 μm 3.5% 2.9% 2.31 g/cm³ 189 Å 4.40 g/cm³ 0.28 g/cm³ 6 3.0% 3 0.86μm 5.5% 8.4% 3.14 g/cm³ 564 Å 4.26 g/cm³ 0.35 g/cm³ 8 4.0%

As will be clear from the data listed in Table 1, the nickel powderproducts prepared in Examples 1 to 3 according to the present inventionhave low contents of coarse particles having a particle size greaterthan a predetermined specific level and fine particles having a particlesize smaller than a predetermined specific level, a narrow particle sizedistribution and a high tap density and therefore, they are alsoexcellent in the packing ability in a conductive paste. Moreover, thefilm formed from such a nickel powder-containing conductive paste has ahigh film density and is uniform and the multilayer ceramic capacitorproduced by using such a nickel powder-containing conductive pasteaccordingly has a low percentage of rejections.

Contrary to this, the nickel powder prepared in Comparative Example 1includes large amounts of coarse particles and fine particles, thepacking ability thereof in a conductive paste is correspondingly low andthe multilayer ceramic capacitor produced by using such a nickelpowder-containing conductive paste accordingly has a high percentage ofrejections. In addition, the nickel powder of Comparative Example 2 hasa low tap density and hence has a low packing ability in a conductivepaste and the multilayer ceramic capacitor produced by using such anickel powder-containing conductive paste accordingly has a highpercentage of rejections. Further, the nickel powder of ComparativeExample 3 has a high content of fine particles and the average particlesize of crystallites present therein is large since the nickel powder isone produced according to a dry method. In addition, the film formedfrom such a nickel powder-containing conductive paste has a scatteredfilm density and the multilayer ceramic capacitor produced by using sucha nickel powder-containing conductive paste accordingly has a highpercentage of rejections.

Industrial Applicability

The nickel powder of the present invention is particularly suitably usedin a conductive paste for use in making a thin and uniform innerelectrode for a multilayer ceramic capacitor without causing any crackformation and/or delamination.

What is claimed is:
 1. Nickel powder characterized in that the rate ofthe nickel particles whose particle size is not less than 1.2 time theaverage particle size as determined by the observation with an SEM isnot more than 5% of the total number of nickel particles, that the rateof nickel particles whose particle size is not more than 0.8 time theaverage particle size is not more than 5% of the total number of nickelparticles and that the tap density of the nickel powder is not less than2.5 g/cm³.
 2. The nickel powder as set forth in claim 1, wherein theaverage particle size of crystallites present in each nickel particle isless than 400 Å.
 3. A conductive paste for use in making a multilayerceramic capacitor characterized in that it comprises nickel powder asset forth in claim
 1. 4. A conductive paste for use in making amultilayer ceramic capacitor characterized in that it comprises nickelpowder as set forth in claim 2.